Contain Virus

Each day, DFS or BFS every uncontained infected component. For a component, collect its distinct adjacent zero cells in a set and count every infected-to-zero edge as one required wall. Select the component with the largest threatened set, mark its cells quarantined, and convert the threatened cells of every other component to infected simultaneously. Repeat until no active region has a threatened cell. Sets matter because several cells in one region can touch the same zero, while wall count deliberately counts each touching edge. Do not spread one region before examining the others, since the specification is a daily simultaneous update. With an R by C grid, repeated simulations are commonly bounded by O((RC) squared) time in the worst case and O(RC) space.